Explosives are substances capable of exerting, by their characteristic high-velocity reactions, sudden high pressures. There are a variety of chemical explosive compounds, each one with characteristics that determine the conditions under which it can advantageously be used. Accordingly, a particular explosive compound may be more desirable for use in one situation than in another, and a different explosive compound will be better suited for use under the other situation's conditions. However, all types of explosives have at least one characteristic in common: they require some sort of activation, by application of one or more externally supplied stimuli such as heat, flame, electrical discharge, impact or shock to initiate their explosive reactions. It nonetheless confirms the diversity of explosives that their sensitivities to the aforementioned stimuli vary from one to another, and vary even for a given explosive under different conditions of temperature, pressure, concentration, density and physical state. Chemical explosives are divided into two main categories, the "low" or "deflagrating" type and the "high" or "detonating" type. The latter are further classified as "primary" or "secondary" high explosives.
The present invention has to do principally with the employment of high explosives in the oil and gas industry. Such high explosives are characterized by their exhibition, when appropriately stimulated, of an explosive reaction which takes place within a high-velocity shockwave known as the "detonation wave" or "reaction shock period". This wave generally propagates at a constant velocity, typically faster than the speed of sound in the high explosive material, depending on the chemistry of the explosive, its density and its physical state. Pressures generated by detonation range up to several millions of psi.
Primary high explosives are used to detonate other high explosives. The reaction in a primary high explosive is typically initiated by a relatively weak mechanical shock or by a spark. Such explosives first burn or deflagrate for a few micro-seconds, then detonate. The treatment and handling of primary high explosives require great care. This is due to their high sensitivity and thus tendency to detonate prematurely, and their tendency towards degradation (through oxidation) when exposed to high temperatures over a period of time.
Secondary high explosives are used in preference to primary high explosives whenever possible. Secondary high explosives are advantageous because they yield higher outputs of energy than primary high explosives. Also, unlike primary high explosives, they can only be detonated in response to (1) a shock wave moving faster than the speed of sound therein, or (2) a deflagration thereof which is transformed into a detonation by confinement of the deflagrating high explosive leading to a sufficient pressure increase accompanied by a sufficiently increased burn rate. In the absence of such stimuli they are relatively stable. Detonation of a secondary high explosive depends in large measure on its confinement, the rate of heat dissipation, and the nature of the explosive itself.
High explosive charges of the secondary type have many different applications in the oil and gas industry; typical uses include perforating a well casing to complete or test a formation, severing tubing in a wellbore, breaking up unretrievable junk downhole and extinguishing fires at wild wells. Due to the time and expense involved in carrying out such operations, and to the power of the explosives, it is essential that the performance of the explosives be as safe and reliable as possible. Furthermore, it is important that secondary high explosive materials be resistant to the extremes of temperature encountered in the typical wellbore environment lest such conditions degrade the operation of those materials.
It is frequently the case that such secondary explosives must be detonated downhole with the aid of a booster, an explosive component ordinarily interposed between two secondary high explosive masses to transmit a detonation from one to the other. Unfortunately, the utilization of conventional boosters has proven to be a long-standing and significant problem in the art, wherein lies the genesis of the present invention.
By way of illustration, a typical use of conventional boosters and the problems engendered by it are discussed below.
It is the ordinary practice of those in the oil and gas industry to utilize secondary high explosives in the form of a plurality of discrete charges set in a tube of appropriate material known as a perforating gun; while it may vary, the gun's length is typically fourteen feet. A detonating cord made of secondary high explosive sheathed in an insulating material such as, for example, lead, glass fibers, aluminum or nylon, is intertwined with the charges from one end of the gun to the other to transmit a detonation from one charge to another, along the entire length of the tube. As those of ordinary skill in the art will appreciate, the intra-gun transfer of detonation from charge to charge works well enough; all of the charges as well as the detonating cord comprise safe, reliable secondary high explosive. Thus, proper operation even under extreme conditions downhole is by and large theoretically ensured.
However, in practice the aforementioned guns are quite often not used singly, but rather, in the form of a string of guns secured to one another in succession. Such a string is lowered down a borehole in order to perform the various functions mentioned above as those for which secondary high explosives are typically used; it is not unusual for the string to be one thousand to two thousand feet long. Initially, the component guns in that string are attached to one another by couplings so that the detonation cords within adjacent guns are in effective abutment, thereby permitting transfer of a detonation from the first gun to the second gun by transmitting the detonation from one detonating cord to the other. But, due to the length of the string and weight of its component guns, as the string is lowered into the hole it stretches, and the component guns pull apart from one another, or "space out", leaving air gaps between adjacent guns.
Therefore, a booster is coupled to each of the opposed ends of the two detonating cords. This is done to enable the transfer downhole of a detonation from the detonating cord of one perforating gun to the detonating cord of an adjacent gun. It is the utilization of boosters in connection with secondary high explosives which raises the basic difficulties addressed by the present invention.
At one time, conventional boosters were all of uniform construction, each including a charge of primary high explosive such as lead azide positioned at the innermost extremity of a metal cup, with an adjacent charge of secondary high explosive. Such boosters were bi-directional in the sense that each could act equally well as a donor or acceptor. However, the use of these boosters was disadvantageous (to say the least) due to their extreme sensitivity to shock or spark.
Subsequently, there were developed the currently employed conventional boosters, which can be classified into two basic types: donor boosters and acceptor boosters.
A donor booster must be capable of transmitting a detonation across a discontinuity such as an air gap. It does this by its own detonation in response to the detonation of an adjacent secondary high explosive mass, the donor booster's detonation yielding a sufficiently high output to enable transmission across the air gap or the like. Because of the output requirements, a donor booster is typically composed of secondary high explosive; such conventional secondary boosters cannot "pick up" a detonation over any discontinuity, for example, an air gap. This means that the donor booster and the detonating cord to which it is coupled must be in intimate contact.
An acceptor booster, on the other hand, is one which will detonate in response to another detonation, i.e., in response to the detonation of a donor booster which may be spaced from the acceptor booster by a discontinuity such as an air gap; the acceptor booster is further capable of detonating another secondary high explosive mass in operative association with it by means of the booster's own detonation. Thus, an acceptor booster "picks up" a detonation from a donor booster, even across a discontinuity, and transmits the detonation to another secondary high explosive mass so as to "continue" the detonation. Therefore, to "continue" the detonation, it is essential that an acceptor booster detonate, and not merely deflagrate.
A conventional acceptor booster usually has two stages; a primary high explosive stage and a secondary high explosive stage. The secondary high explosive stage is located adjacent a detonating cord, and the primary high explosive stage is located adjacent the secondary high explosive stage. The primary high explosive stage "picks up" a detonation, e.g., across an air gap (due to heat, shock or the like stimulus generated by the detonation) and detonates itself; such detonation in turn causes the secondary high explosive stage to detonate which in turn causes the above-mentioned detonating cord (or other adjacent secondary high explosive mass) to detonate.
From the foregoing, the necessity of placing conventional donor and acceptor boosters in proper sequence, and the drawback of this sequencing requirement, are apparent. If the donor booster is placed out of sequence in the position of the acceptor booster, a detonation will not be accepted by the donor booster should any sort of discontinuity be interposed between it and the detonation--as is virtually always the case. This will terminate the progression of the detonation. The problem is a substantial one due to the fact that conventional donors and acceptors are normally housed in look-alike aluminum containers, thereby creating a probability that a donor will be placed where an acceptor ought to be.
An additional significant drawback to the use of conventional boosters is the incorporation therein of the primary high explosive stage, for instance lead azide or a related compound. Lead azide, and other primary high explosives, are highly sensitive to shock and heat and accordingly are subject to possible premature detonation with self-evident detrimental consequences. Additionally, the exposure of lead azide to high temperatures over a period of time will result in its degradation through oxidation. For example, exposure of lead azide to a temperature of 437.degree. F. or more for a period of 100 hours or more will result in a loss of effectiveness. This shortcoming of lead azide is particularly significant when working in deep wells. As the string of perforating guns is made up at the well-head it is gradually lowered down the borehole; the deeper the well the longer the string, and thus the longer the dwell-time of the string downhole (until make-up of the entire string is completed) prior to detonation. Furthermore, the deeper the well the higher the temperature encountered at its lower depths. These factors combine to increase time-at-temperature exposure to a dangerously high degree. It is therefore desirable to avoid the use of lead azide and other similar primary high explosives whenever possible.
A secondary high explosive booster capable of both receiving a detonation across a discontinuity and continuing the detonation downline would confer clear advantages, such as obviation of the sequencing requirement of conventional acceptor and donor boosters, high stability and high output. It would eliminate the previously discussed problems associated with use of primary high explosives. Provision of such a booster would be a highly desirable advance over the current state of technology.